Code Division Multiple Access (CDMA) communication systems may be used to provide wireless communications between a base station and one or more field units. The base station is typically a computer controlled set of transceivers that are interconnected to a land-based public switched telephone network (PSTN). The base station includes an antenna apparatus for sending forward link radio frequency signals to the field units. The base station antenna is also responsible for receiving reverse link radio frequency signals transmitted from each field unit.
Each field unit also contains an antenna apparatus for the reception of the forward link signals and for transmission of the reverse links signals. A typical field unit is a digital cellular telephone handset or a personal computer coupled to a cellular modem. In CDMA cellular systems, multiple field units may transmit and receive signals on the same frequency but with different codes, to permit detection of signals on a per unit basis.
The most common type of antenna used to transmit and receive signals at a field unit is a mono-pole or omni-antenna. This type of antenna consists of a single wire or antenna element that is coupled to a transceiver within the field unit. The transceiver receives reverse link signals to be transmitted from circuitry within the field unit and modulates the signals onto the antenna element at a specific frequency assigned to that field unit. Forward link signals received by the antenna element at a specific frequency are demodulated by the transceiver and supplied to processing circuitry within the field unit.
The signal transmitted from a monopole antenna is omni-directional in nature. That is, the signal is sent with the same signal strength in all directions in a generally horizontal plane. Reception of a signal with a monopole antenna element is likewise omni-directional. A monopole antenna does not differentiate in its ability to detect a signal in one direction versus detection of the same or a different signal coming from another direction.
A second type of antenna that may be used by field units is described in U.S. Pat. No. 5,617,102. The system described therein provides a directional antenna comprising two antenna elements mounted on the outer case of a laptop computer. The system includes a phase shifter attached to the two elements. The phase shifter may be switched on or off in order to affect the phase of signals transmitted or received during communications to and from the computer. By switching the phase shifter on, the antenna transmit pattern may be adapted to a predetermined hemispherical pattern which provides transmit beam pattern areas having a concentrated signal strength or gain. The dual element antenna directs the signal into predetermined quadrants or hemispheres to allow for large changes in orientation relative to the base station while minimizing signal loss.
Yet another type of antenna is a scanning directional antenna that employs at least one central active antenna element and multiple passive antenna elements. By changing impedance settings between the passive antenna elements and a ground plane, a beam produced by the directional antenna can be scanned in a fixed number of directions related to the number of passive antenna elements. An example of such a directional antenna is described in U.S. Pat. No. 5,767,807 by Pritchett.